In portable terminals such as portable telephones, it has become common to install a plurality of antenna elements for communication on different frequency bands in several wireless schemes (such as the GSM (Global System for Mobile Communications) mode or WCDMA (Wideband Code Division Multiplex Access) mode according to 3GPP (3rd Generation Partnership Project)). MIMO (Multi-Input/Multi-Output) communication technology that is capable of using space diversity to expand transmission bands is now receiving attention.
MIMO communication is a communication technology in which a plurality of antenna elements are provided on both a transmitting side and receiving side that carry out wireless communication and in which data stream are spatially multiplexed to transmit from the transmitting side to the receiving side. Where M and N are integers equal to or greater than 2, a plurality of transmission data stream are subjected to spatial and temporal encoding for multiplexing, distributed to M transmission antennas, and transmitted on channels on the transmitting side. In contrast, received signals that are received by N reception antennas via channels on the receiving side are subjected to spatial and temporal decoding based on the channel characteristics to separate a plurality of data streams from the received signals. In this way, received data can be obtained without crosstalk between data streams. Ideally, MIMO communication enables a number of independent transmission paths to be obtained that is equal to the number of, among the transmission antennas and reception antennas, the antennas of which there are fewer, i.e., the number represented by min{M, N}. This number is called the spatial multiplicity or the spatial multiplexing number. In MIMO communication, the separation of data series is implemented reliably through the low correlation between the antenna elements.
MIMO technology is a technology that, by thus enabling the transmission of different data series from each of a plurality of antenna elements and then separating data series that have been simultaneously transmitted from each of transmitting-side antenna elements on a receiving side that is equipped with a plurality of antenna elements, raises the transmission capacity without expanding the frequency band. According to MIMO technology, transmission capacity can be increased according to the number of antennas. If the signal power is S and the noise power is N, the inherent performance of MIMO communication can be drawn out to its maximum when the SN ratio (the signal-to-noise ratio) is great, such as when the noise power N is low.
The MIMO communication scheme is used in, for example, wireless LAN (Local Area Network) standard IEEE 802.11n, LTE (Long Term Evolution) of UMTS (Universal Mobile Telecommunication System) that is the next-generation mobile communication standards and currently in development in 3GPP (3rd Generation Partnership Project), and in mobile WiMAX that takes the IEEE 802.16e standard as a basis.
However, when the MIMO communication scheme is applied to a compact device such as a portable telephone terminal, the small size of the device body compels the mounting of a plurality of antenna elements in an extremely confined space, thereby complicating the reduction of the correlation among the antenna elements which severely affects the inherent performance of MIMO communication.
The MIMO communication scheme is described in greater detail hereinbelow. The MIMO communication scheme is here described for a case in which the number of transmission antennas is two and the number of reception antennas is two, i.e., 2×2 MIMO. In addition, a link by which signals are transmitted from a base station to a portable terminal is referred to as a downlink, and a link by which signals are transmitted from a portable terminal to a base station is referred to as an uplink.
As shown in FIG. 1, when data series A (not shown) and data series B (not shown) are transmitted simultaneously from two transmission antennas T-ANT1 and T-ANT2, respectively, each of two reception antennas R-ANT1 and R-ANT2 receives a different data series in which each of data series A and data series B have been multiplexed. Because two antennas have been provided for each of transmission antennas and reception antennas, four (=2×2) wireless channels are formed between the transmitting side and receiving side. In the figure, these wireless channels are indicated by arrows. On the receiving side, data series A and data series B that have arrived on the receiving side from the two transmission antennas T-ANT1 and T-ANT2 are separated by estimating the channel fluctuation, i.e., channel transfer functions h11(t), h12(t), h21(t) and h22(t) in these four channels. In order to enable unique identification of the data series from each transmission antenna on the receiving side and to enable the estimation of channel fluctuation h11(t), h12(t), h21(t) and h22(t), a pilot signal is embedded in the transmission data series from each transmission antenna.
It is here assumed that when the number of transmitting-side antennas is T and the number of receiving-side antennas is R, the configuration is represented by “T×R.” According to the specifications of 3GPP LTE, 2×2, 4×2 and 4×4 configurations are defined as the configurations of MIMO that are used in downlink data transmission. Accordingly, in an LTE portable terminal to which MIMO technology has been applied, two or four antenna elements must be installed within the terminal.
However, if λ is the wavelength in free space of the frequencies used in communication, antenna elements in a terminal must be separated by at least λ/2 in order to render the reduction in performance negligible in MIMO communication. In consideration of this condition, the installation of two or more antenna elements for MIMO communication inside the case of a portable terminal requires still more space within the portable terminal. The installation of four antenna elements in a portable terminal is particularly difficult.
Because a portable terminal generally already has a plurality of antenna elements for different communication schemes as previously described, mounting a plurality of antennas for MIMO use in such a portable terminal necessitates not only the provision of a plurality of antenna elements in an extremely confined space but also an increase in the size of the portable terminal to both prevent interference between the antenna elements and maintain low correlation. When the 2×2 MIMO communication scheme is to be applied, the addition of still more antenna elements to the portable terminal to bring out the inherent capabilities of MIMO communication is problematic.
Then JP-A-2008-205904 (Patent Literature 1) discloses the use of a plurality of portable terminals and causing these terminals to operate in synchronization in order to enable uplink MIMO communication. Because the stream is merely divided and transmitted from different antenna elements on the transmitting side, the use of a plurality of portable terminals as the transmitting side in MIMO communication is easy. However, JP-A-2008-205904 (Patent Literature 1) makes no disclosure regarding the configuration that is necessary for using a plurality of portable terminals to implement MIMO communication as the receiving side that requires complex processing such as the estimation of channels and the separation of the stream. JP-A-2007-140590 (Patent Literature 2) discloses an example of a communication system in which synchronous control is realized regarding the operation of processing among a plurality of communication terminal devices.